Direct Solar Thermal Hydrogen Processes

The problems with materials and separations at such a high temperature make direct decomposition not attractive at this time. The production of hydrogen by direct thermal splitting of water generated a considerable amount of research during the period 1975-1985. Fletcher and co-workers stressed the thermodynamic advantages of a one-step process with heat input at as high a temperature as possible.25-27 The theoretical and practical aspects were examined by Olalde,28 Lede,29-31 Ounalli,32...

Renewable Energy and the Terawatt Challenge

Technological advancement and a growing world economy during the past few decades have led to major improvements in the living conditions of people in the developed world. However, these improvements have come at a steep environmental price. Air quality concerns and global climate impact constitute two major problems with our reliance on fossil energy sources. Global warming as a result of the accumulation of greenhouse gases such as CO2 is not a new concept. More than a century ago, Arrhenius...

Hydrogen as a Fuel of the Future

Jules Verne appears to be one of the earliest people to recognize, or at least articulate, the idea of splitting water to produce hydrogen (H2) and oxygen (O2) in order to satisfy the energy requirements of society. As early as 1874 in The Mysterious Island, Jules Verne alluded to clean hydrogen fuels, writing Fig. 2. The terawatt renewable energy challenge the energy mix has to switch from the panel on the left to the panel on the right to cap CO2 levels at safe limits. Data from the...

The Spectral Distribution of the Sun as a Radiation Source

In this chapter we briefly describe the solar spectral distribution, or distribution of energy with respect to wavelength, over the region of the electromagnetic spectrum of use to renewable energy systems. The sun radiates energy at wavelengths ranging from the X-ray and gamma ray spectral region out into the very long wavelength radio spectral region. We will restrict our discussion, for the most part, to solar energy in the wavelength region between the ultraviolet (UV) of wavelength 250...

Types of Approaches

Hybrid Generation Solar Cell

Figure 1 illustrates the interfacial energetics involved in the photoelectrochemical evolution of H2. Thus, the electronic energy levels in the semiconductor and in the contacting solution are shown on a common diagram. In a semiconductor, the filled electronic levels (valence band or VB) and the empty levels (conduction band or CB) are separated by a forbidden gap, namely, the band gap energy, Eg.98-100 Photoexcitation of the semiconductor with light of energy equal to or exceeding Eg (i.e.,...

Following in Wind Energys Footsteps

Wind energy is one renewable energy technology developed successfully by mechanical engineers. In the 1970s, wind systems and PV systems started out on nearly the same footing only a few experimental systems for each were installed around the world. Today, there are roughly 10 times more wind energy systems than PV systems installed 50,000 MW of wind systems versus 5,000 MW of PV systems. Why were wind energy technologies able to surpass PV systems One reason was that wind developers were able...

Future Outlook and Concluding Remarks

A hybrid solar thermal electrochemical process combines efficient photovoltaic devices and concentrated excess sub-bandgap heat into highly efficient elevated temperature solar electrolysis of water to generate H2 fuel. Efficiency is further enhanced by excess super-bandgap and non-solar sources of heat but diminished by losses in polarization and photo-electrolysis power matching. As also shown earlier in Chapter 4 and elaborated further here, solar concentration can provide the high...

Theory of Hybrid Solar Hydrogen Generation

Thermally assisted solar electrolysis consists of (i) light harvesting, (ii) spectral resolution of thermal (sub-bandgap) and electronic (super-bandgap) radiation, the latter of which (iiia) drives photovoltaic or photoelectrochemical charge transfer V(i o), while the former (iiib) elevates water to temperature T, and pressure, p finally (iv) V(iH2o) driven electrolysis of H2O(T,p). A schematic representation for this solar thermal water electrolysis (photothermal electrochemical water...

Utilization of Solar Spectral Regions Spectral Response of Materials

Air Mass Solar Spectrum

Why this extended discussion of the solar spectral distribution The primary reason is provided by the example discussing the sensitivity of our eyes, in the previous Fig. 7. Temperature and color relationships for various natural and artificial sources of optical Fig. 7. Temperature and color relationships for various natural and artificial sources of optical Fig. 8. Decreasing direct beam spectral irradiance as aerosol optical depth at 500 nm increases from 0.05 top curve to 0.04 bottom curve...

Contributors

National Renewable Energy Laboratory 1617 Cole Blvd., Golden, CO 80401 marie_ghiradi nrel.gov National Renewable Energy Laboratory NREL MS3911 Ph 303-384-7091, F 303-384-7055, Kevin_Harrison nrel.gov National Renewable Energy Laboratory 1617 Cole Blvd., Golden, CO 80401 johanna_levene nrel.gov 4201 Wilson Blvd., Arlington, VA 022230 Chemistry Department 100 Morrissey Boulevard University of Massachusetts, Boston, MA 02135-3395 Ph 617-287-6156, stuart.licht umb.edu Department of Chemistry and...

Higher and Higher Conversion Efficiencies

With the advent of funding from the DOE in the late 1970s and early 1980s came plans and goals to develop PV technologies through improving performance efficiency , reducing cost, and assuring reliability of operation. CPV systems offered the possibility of lower cost because expensive solar cells are replaced with less costly structural steel holding mirrors or lenses. However, early CPV systems showed the importance of optical efficiencies as optical losses typically reduced the CPV system...

Nitrides Oxynitrides and Oxysulfides

We have seen that introduction of nitrogen into the TiO2 lattice has a favorable effect in terms of sensitizing it to the visible range of the electromagnetic spectrum Table 6 . The line between doping and new phase formation is one of degree and the studies on nitridation of a given parent oxide exemplify this point. Thus the band gap of Ta2O5 shrinks from 4.0 eV to 2.1 eV by nitriding it in a NH3 atmosphere to yield Ta3N5.476 This material evolves H2 and O2 under visible irradiation k lt 600...

Natures Photosynthetic Machinery

Natural photosystems have been extensively studied and the machinery of photosynthesis found to be highly modular, organized on multiple scale levels and compart-mentalized.32 Importantly, all natural photosystems are molecule-based and their function can be understood both at the schematic level and, in many cases, at the molecular-level. Highly specialized components interact in controlled manner to ultimately deliver a product reduction equivalents and to effectively deal with waste...

Electrolysis of Water

Hydrogen Oxygen Separation

Personally, I think that 400 years hence the power question in England may be solved somewhat as follows. The country will be covered with rows of metallic windmills working electric motors, which in their turn supply current at a very high voltage to great electric mains. At suitable distances there will be great power stations where during windy weather the surplus power will be used for the electrolytic decomposition of water into hydrogen and oxygen In times of calm, the gases will be...

Free Energ Engen

Rev. 1992, 92, 435-461. 2. Cole-Hamilton, D. J. Bruce, D. W. In Comp. Coord. Chem. V. 6 Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds. Pergamon Books Ltd Maxwell House, NY, 1987 Vol. 6. 3. Holten, D. Bocian, D. F. Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57-69. 4. Kodis, G. Liddell, P. A. de la Garza, L. Clausen, P. C. Lindsey, J. S. Moore, A. L. Moore, T. A. Gust, D. J. Phys. Chem. A. 2002, 106, 2036 - 2048. 5. Fukuzumi, S. Imahori, H. In Electron Transfer...

Perovskite Titanates and Related Oxides

Photoelectrochemical Cell

Perovskites have the general formula, ABX3, with SrTi 3 being a prototype. They contain a framework structure containing corner-sharing Ti 6 octahedra with the A cation in twelve-coordinate interstices.384,385 Several hundred oxides have this struc Fig. 6. Twin-compartment photoelectrochemical cell for the photocatalytic generation of H2 from water using electrodeposited p-Cu2O from Ref. 382 . TCO is the transparent conducting oxide substrate and A is an electron donor in the anode compartment....